1. Field of the Invention
The present invention relates to a mask pattern generating method and pattern generating apparatus suitable when desiring to form fine patterns on a wafer substrate at a high resolution from a transmittance light of a photo mask using an exposure device in the process of manufacturing a highly integrated semiconductor device etc.
2. Description of the Related Art
The photo masks used in the process of manufacturing semiconductor devices are comprised of a light blocking film formed on a glass substrate. In a pattern forming process of semiconductor elements, the light blocking pattern on the photo mask is projected on a photo resist coated on the wafer surface and then exposed. The light blocking pattern on the photo mask is obtained by converting designed CAD data to drawing apparatus data and faithfully patterning it. The photo mask pattern is precisely transferred onto the wafer by a semiconductor photolithographic process.
In the photolithographic process in the process of manufacturing a semiconductor device, a high resolution beyond the resolution limit determined by the wavelength of the light is required because it is necessary to form the pattern close to an exposure wavelength. Accordingly, the phase shift method has recently been used as a lithographic technique which can form finer patterns than the exposure wavelength.
Below, the principle of a spatial frequency modulation type phase shift method will be explained.
FIGS. 16A and 16B are views of the principle of the phase shift method compared with a method of the prior art, and FIGS. 17A and 17B are views of Fourier spectrum of the phase shift method and the method of the prior art.
Here, assume that a period of light intensity transmittance is close to the resolution limit of a one-dimensional period pattern having 1/xcexd0. Since the light transmitting object is the mask pattern whose size is close to the resolution limit, when considering only the sinusoidal basic frequency component, the amplitude of the light passing through the photo mask can be approximated as follows:
Mask of the related art:
T(x)=|cos 2xcfx80xcexd0x|xe2x80x83xe2x80x83(1-1)
Phase shift mask:
T(x)=cos 2xcfx80xcexd0xxe2x80x83xe2x80x83(1-2)
In the method of the related art shown in FIG. 16A, the light passing through the photo mask was divided into a 0-order diffracted light proceeding straight along the light axis and a xc2x11st-order diffracted light having an angle of xcex8 (sin xcex8=xcexd0xcex) with respect to the light axis and strikes a projection lens. On the other hand, as shown in FIG. 16B, in the phase shift method, the light passing through the phase shift mask is separated into a xc2x11st-order diffracted light having an angle of xcex8/2 with respect to the light axis and strikes the projection lens. In both cases, only the refracted light passing through the inside of the projection lens contributes to form an image.
The Fourier spectrum formed on an iris plane of the projection lens is expressed as follows from the Fourier transform of the formulas 1-1 and 1-2.
Mask of the related art:
F(xcexd)=(4/xcfx80){xcex4(xcexd)/2+[xcex4(xcexd+xcexd0)]+xcex4(xcexdxe2x88x92xcexd0)]/2+ . . . }xe2x80x83xe2x80x83(2-1)
Phase shift mask:
F(xcexd)=(xc2xd)[xcex4(xcexd+xcexd0/2)+xcex4(xcexdxe2x88x92xcexd0/2)]xe2x80x83xe2x80x83(2-2)
As shown in FIGS. 17A and 17B, in a transmittance type mask of the related art not considering phases, there are spectrum components at xcexd=0 and xc2x1xcexd0, while in a phase shift mask, this depend only on xcexd=xc2x1xcexd0/2. Namely, the basic spectrum obtained from the phase shift mask lies at a position of half of the spectrum obtained by the transmittance type mask. This is equivalent to the spatial frequency of the mask pattern becoming half. The projection lens of a stepper acts as a low-pass filter for transferring only the spatial frequency component smaller than a proper frequency (Eigen frequency) xcexdc (=NA/xcex). When considering a case where the spatial frequency xcexd0 of a not transferred fine pattern is xcexdc less than xcexd0xe2x89xa6284 c, in the case of a transmittance type mask, the spectrum component of xc2x1xcexd0 does not pass, so no contrast of the image can be obtained. On the other hand, a phase shift mask transfers the basic spectrum xcexd=xc2x1xcexd0/2 so forms a pattern image on the image plane. This is the characteristic of a spatial frequency modulation type phase shift mask which has the largest effect of improving the resolution.
The basic layout structure of the spatial frequency modulation type phase shift mask uses, as shown in FIG. 16B, two phases of 0 degree and 180 degrees and arranges shifters in order that apertures of the mask alternately become inverse in phase. As a shifter, as shown in FIG. 16B, there are shifters stacked on films different from the light blocking film and shifters formed by etching a glass surface to impart a shifter function.
This shifter arrangement corresponds to a two-color problem of coloring a flat map by two colors. The shifter phases have to be arranged alternately. Therefore, in a complicated interconnection layout, in principal it is impossible to avoid an arrangement discrepancy (phase mismatch) where the shifter phases cannot be alternately inverted.
As a first prior art, Ohi et al. in the article xe2x80x9cMethod of Design of Phase-Shifting Mask Utilizing Compactorxe2x80x9d, JJAP, Vol. 33 (1994), No. 12B, pp. 6774-6778, discusses a technique for avoiding phase mismatch using layout compression (compaction) in a phase shift method using the phase difference of transmittance light between patterns formed.
FIG. 18 is a view of the principle of a so-called Levenson type phase shift utilizing a negative resistxe2x80x94a premise of the first prior art.
In the photo mask shown in FIG. 18, chrome apertures provided at the pattern forming locations are alternately arranged with shifters (glass etching trenches) of a 180-degree phase and without trenches of a 0-degree phase. Accordingly, in this phase shift method, since the phase difference of the light passing through the pattern itself is utilized, a negative resist in which the pattern transferring locations remain without being dissolved in the developing solution is necessary. In addition to the fact that obtaining a practical negative resist is difficult, it is necessary that the pattern layout itself be a pattern which generates a phase difference. Therefore, while this is effective with simple lines and spaces, there is no phase shift effect with an isolated pattern. Accordingly, in the first prior art, even though it is possible to avoid a phase mismatch, there is the disadvantage that there are many constraints in the predicted resist materials and pattern preparation.
Therefore, in recent years, studies have been made on a technique based on multiple exposures including phase shift exposures by a mask giving a phase difference to the light passing through the two sides of the fine patterns to be formed and ordinary exposures for removing the unnecessary patterns generated due to the same (hereinafter referred to as the phase shift multiple exposure method). This second prior art is discussed by Tamechika et al. in the article xe2x80x9cAutomatic Generation of Phase-Shifting Mask Patterns Using Shifter-Edge Linesxe2x80x9d, MICRO-AND-NANO-ENGINEERING ""97xe2x80x9d.
FIGS. 19A and 19B are views of the principle of a Levenson type phase shift utilizing a negative resist in the phase shift multiple exposure method.
In the phase shift mask shown in FIG. 19A, apertures provided at the sides of fine patterns (chrome patterns) are alternately formed with shifters of 180-degree phase (glass etching trenches) and without trenches of 0-degree phase. The peripheries are covered with chrome because a positive resist is used. In the second exposure (ordinary exposure), not illustrated peripheral interconnection portions are image-developed while light is blocked from the fine patterns by using a photo mask shown in FIG. 19B.
In the second prior art using the phase shift multiple exposure method, the design layout pattern is restricted to be always parallel. However, actual layout design patterns are not necessarily like that. The patterns of devices which can be prepared by this method are therefore limited.
Also, in the second prior art, due in part to the fact that the shifters are arranged based on the premise of a fixed mask layout pattern, there are locations where phase discrepancies occur. For correction, it is necessary to return once again to the cell design stage to correct them manually. In this way, in the design of a phase shift mask, with the method of determining the arrangement of shifters after the preparation of the layout, when there is a discrepancy in the layout, if a designer intervenes and corrects the contradictory location, many procedure steps are required for the arrangement of shifters and determination of the phases and the design efficiency becomes extremely poor.
Furthermore, with fine patterns formed by phase shift patterns, the rough and fine degree due to the distances between the patterns sometimes cause variations in the line width of the patterns formed. In order to reduce this, changing the line widths of the fine patterns in accordance with the distance between patterns has been studied in the past. In patterns so fine that as to reach the limits of control in the manufacturing process of a photo mask, however, the result of the correction would be outside the limit of the line width in the manufacturing process of the photo mask and there is a possibility that photo mask could not be produced.
An object of the present invention is to provide a method and apparatus for generating mask patterns suitable for multiple exposures which enables automatic arrangement of shifters without any discrepancies and can tremendously improve the design efficiency in the design of phase shift masks and which corrects the dependency of line width on rough and fine degree due to the distances between the patterns of the fine processing patterns utilizing a phase shift.
The present inventor engaged in intensive studies to solve the above disadvantages of the prior art. As a result, in the method of layout of a spatial frequency modulation type phase shift mask pattern, as a pattern generating method suitable for the phase shift multiple exposure method forming only locations where control of the line width is difficult by phase shift and exposing the remaining portions by an ordinary mask, he newly proposed the technique of arranging the phase shifters so as to be able to form the two adjacent portions of the portions where control of the line width is difficult by transmittance light of different phase differences and discovered that this enabled automation of the generation of phase shift patterns and determination of the phases and enabled higher speed processing even in changes of patterns for eliminating locations of phase discrepancies.
Namely, to solve the above disadvantages of the prior art and to attain the above object, the pattern generating method of the present invention provides a pattern generating method for a mask pattern including a plurality of fine patterns and a plurality of phase shift patterns arranged at the two sides of said fine patterns in a direction of fine line width to eliminate light interference due to a phase difference of light passing through, wherein shapes and phases of the plurality of phase shift patterns are determined based on positional relationships of the plurality of fine patterns and the phases are determined to give a phase difference of 180 degrees at the two sides of said fine patterns.
Preferably, when there is a phase mismatch giving a phase difference of 0 degree, the pattern is changed to eliminate the phase mismatch.
Preferably, the shapes and phases of the phase shift patterns are determined by the following routine.
First, the plurality of fine patterns are extracted from already designed element shape patterns. Unit patterns of at least a predetermined width required for canceling the light interference are arranged at the two sides of each of the extracted plurality of the fine patterns in the direction of fine line width (for example, so that no clearance can be formed at the opposite sides of the fine patterns while changing the width in the direction of fine line width). Further, the plurality of phase shift patterns are generated using the unit patterns (for example, by obtaining the graphical OR""s of the arranged unit patterns).
Further, the phases are determined while searching for adjoining phase shift patterns straddling the fine patterns so that phase differences between adjoining phase shift patterns successively become 180 degrees.
In this pattern forming method, the shapes and phases of the phase shift patterns are not determined at the stage of designing the patterns (element shape patterns) which determines the actual element shapes. The shapes and phases of the phase shift patterns are determined based on the positional relationships of the plurality of fine patterns to be formed by high resolution by phase shift.
For example, the fine pattern locations are extracted from the parts of the element shape patterns other than the phase shift patterns and the phase shift patterns are arranged at predetermined widths giving at least a phase shift effect at the two sides in the direction of fine line width to determine the shapes. The phases of the phase shift patterns are successively determined. For example, any fine pattern is taken note of and the phases of the phase shift patterns around the noted pattern are determined so that the surrounding phases differ by 180 degrees. Next, another fine pattern sharing a pattern for which the phase is determined is taken note of and the phase of the remaining phase shift pattern is determined to differ by 180 degrees. The phases are successively continuously determined so as to determine the phases of all of the phase shift patterns.
When a phase mismatch giving unchanged phases of the two sides of a fine pattern arises at the final stage of the phase determination, the phase mismatch is eliminated by changing the pattern.
For example, the design rule is changed to compress (or expand) the entire pattern layout. At this time, if a constraint such as for example not to apply compression to phase mismatch locations etc. is given in advance, along with the design rule of the surrounding patterns and alignment margin etc. becoming smaller, the required width of the phase shift patterns becomes smaller. On the other hand, since the locations of the phase shift patterns on which constraints are placed are not compressed in size, a margin of space is generated. As a result, it becomes possible, for example, to divide a phase shift pattern by 2 to a phase difference xcfx80 and therefore possible to eliminate the phase mismatch. Also, the methods of searching for a phase shift pattern able to be divided by 2 to change the pattern and of securing a space able to be divided by 2 by separating the fine patterns can be adopted. Of course, when the location where a phase mismatch occurs is not a fine pattern, the phase mismatch may be left as it is.
Also, the relationship between the phase pattern locations and the adjoining phase patterns is handled as graph data. This graph data is used as the basic data for automatically determining the remaining phases when the phase of one location is determined. Accordingly, it is easy to determine a phase for a phase shift pattern which is linked by the graph even without actually searching for the pattern. Also, the existence of a phase mismatch can be detected instantaneously in advance and therefore higher speed processing becomes easy. Further, even when changing a pattern, corrections can be made on the graph and the data used to assist actual changes in the pattern and therefore pattern changes can be facilitated.
On the other hand, the pattern forming method of the present invention for easing the rough and fine degree of the line width in the phase shift multiple exposure method is a pattern generating method for generating a mask pattern used when forming a single layer by a plurality of exposures including high resolution exposure using a plurality of fine patterns and a plurality of phase shift patterns arranged at the two sides of the fine patterns in the direction of fine line width for canceling light interference due to a phase difference of passing light and ordinary exposure for locations other than the fine patterns, wherein when generating a mask pattern used for said ordinary exposure, a size of a light blocking pattern on said mask for ordinary exposure superposed at a position corresponding to a said fine pattern on said mask for high resolution exposure is changed in a direction reducing a line width difference after image-development caused by the density of said fine patterns.
The easing of the line width difference due to the rough and fine degree of the fine patterns was limited with correction only on the phase shift mask of the related art, but in this pattern generating method, it is possible to reduce or correct the line width difference after image-development caused by the rough and fine degree of the fine patterns in the second and later ordinary exposures. Generally, the fine patterns of the phase shift masks are already made fine to the limit of manufacture of a photo mask, however, in the masks for the second and later ordinary exposures, there is often a margin for change of the size. In this method, it is possible to reduce or correct the difference in line width between the fine patterns while avoiding constraints derived from the limits of manufacture of the photo mask.
The pattern generating apparatus of the present invention includes, as means for working the pattern generating method of the present invention explained above, an arrangement determining means for determining positions and shapes of said plurality of phase shift patterns based on positional relationships of said plurality of fine patterns and a phase determining means for determining phases of said plurality of phase shift patterns based on positional relationships of said plurality of fine patterns and so that a phase difference at the two sides of said fine shift patterns becomes 180 degrees. Further, preferably, it has a pattern changing means for changing a pattern to cancel said phase mismatch when a phase mismatch occurs resulting in a phase difference of 0 degree.